Sintered ferrite magnets are used for various applications such as various motors, electric power generators, speakers, etc. As typical sintered ferrite magnets, Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) having hexagonal, M-type magnetoplumbite structures are known. These sintered ferrite magnets are produced by a powder metallurgy method from starting materials, for example, comprising iron oxide and carbonates of strontium (Sr) or barium (Ba), etc., at relatively low cost.
From the environmental point of view, etc., higher-performance sintered ferrite magnets are recently desired to provide electric parts for automobiles, electric appliances, etc. with smaller sizes and weights as well as higher performance. Particularly desired for motors used in automobile electric parts are sintered ferrite magnets having high coercivity HcJ (hereinafter simply referred to as “HcJ”), by which demagnetization does not easily occur by a demagnetizing field when they are made thinner, while keeping high residual magnetic flux densities Br (hereinafter simply referred to as “Br”).
To improve the magnetic properties of sintered ferrite magnets, technologies have been proposed to improve HcJ and Br by substituting part of Sr with rare earth elements such as La, etc., and part of Fe with Co in the above Sr ferrites (see, for example, JP 10-149910 A and JP 11-154604 A).
Because of excellent magnetic properties, Sr ferrites with part of Sr substituted by rare earth elements such as La, etc., and part of Fe substituted by Co, etc., which are described in JP 10-149910 A and JP 11-154604 A, and hereinafter simply referred to as “SrLaCo ferrites,” have got used in various applications in large volume in place of conventional Sr ferrites and Ba ferrites. However, further improvement of magnetic properties is desired.
Also known as the sintered ferrite magnets are Ca ferrites, in addition to the above Sr ferrites and Ba ferrites. It is known that the Ca ferrites have stable structures represented by CaO—Fe2O3 or CaO-2Fe2O3, forming hexagonal ferrites by the addition of La. However, their magnetic properties are on the same level as those of conventional Ba ferrites, not sufficiently high.
Japanese Patent 3,181,559 discloses a Ca ferrite with part of Ca substituted by rare earth elements such as La, etc. and part of Fe substituted by Co, etc. for improved Br, HcJ, and temperature characteristics of HcJ, which is hereinafter simply referred to as “CaLaCo ferrite.” Japanese Patent 3,181,559 describes that the anisotropic field HA (hereinafter simply referred to as “HA”) of CaLaCo ferrites is 20 kOe or more, at most 10% or more higher than that of Sr ferrites.
However, the CaLaCo ferrite of Japanese Patent 3,181,559 has Br and HcJ on the same level as those of SrLaCo ferrites and extremely poor rectangularity Hk/HcJ (hereinafter simply referred to as “Hk/HcJ”), though they have higher HA than that of SrLaCo ferrites. Thus, the CaLaCo ferrite fails to satisfy both requirements of high HcJ and high Hk/HcJ, being not used in various applications such as motors, etc.
To improve the magnetic properties of the above SrLaCo ferrites and CaLaCo ferrites, the following proposals have been made.
JP 11-195516 A proposes an improvement of the production methods of SrLaCo ferrites proposed by JP 10-149910 A and JP 11-154604 A, which comprises adding La and Co to calcined Sr ferrite powder, and molding and sintering the resultant mixture. It describes that this method provides sintered magnets having two different Curie temperatures (Tc) as well as high rectangularity, with a lower Co content.
JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 propose improvements of the performance of CaLaCo ferrites proposed by Japanese Patent 3,181,559. Specifically, JP 2006-104050 A proposes a technology of optimizing the atomic ratios and molar ratio n of constituent elements, with La and Co added at particular ratios, WO 2007/060757 proposes a technology of substituting part of Ca with La and Ba, and WO 2007/077811 proposes a technology of substituting part of Ca with La and Sr.
JP 11-195516 A has improved the magnetic properties of the SrLaCo ferrites proposed by JP 10-149910 A and JP 11-154604 A, and JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 have improved the magnetic properties of the CaLaCo ferrites proposed by Japanese Patent 3,181,559. However, as demand for higher performance is increasingly higher recently, further improvement of magnetic properties is desired.